Each generator shows an air gap, which is located between stator elements and rotor elements of the generator.
The rotor elements contain permanent magnets for example, while the stator elements contain stacked laminate plates, which support at least one winding of the stator coil.
The air gap should be relatively small to ensure a high efficiency of the generator. Thus the air gap should stay in a range of only a few millimeters. For generators, like direct drive or directly driven generators, this is very difficult due to their size. Direct drive generators show a diameter of several meters.
Rotor elements and stator elements are arranged opposite to each other, thus the air gap has to ensure that they do not come into contact while the generator is at operation. Thus the air gap should be very small to ensure the efficiency of the generator on the one hand while a certain width of the air gap is needed to prevent mechanical damages.
Especially for a direct drive generator it is difficult to keep the air gap in a range of only a few millimeters. This requires therefore very rigid, massive and heavy support structures for the stator elements and for the rotor elements.
The air gap of a generator is determined:                by tolerances of the permanent magnets, which are part of the rotor,        by tolerances of the stacked laminate-plates, which are part of the stator, and/or        by tolerances of the coil-windings, which are part of the stator-coil.        
Also other elements of the generator contribute to the dimensions of the air gap. The air gap is designed in a way that the elements of the rotor and of the stator do not get in contact, while the rotor rotates around its dedicated rotational axis.
Different bearing arrangements for a direct drive generator are known. One of them is the so called “two bearing” arrangement. This arrangement is well known from document EP 1 641 102 A1 and from document U.S. Pat. No. 6,483,199 B2 for example.
According to these documents the rotor of the generator is connected to the shaft of the wind turbine. The shaft itself is supported by two bearings. The stator of the generator is attached to one side via a bearing to a so called “stationary inner shaft”. Thus the rotor rotates relatively to the stator around the stationary inner shaft.
Due to the one-sided support of the stator it is difficult to keep the air gap constant or at least nearly constant. Also gravity acts on the large generator, influencing the air gap. The rotor-components also influence the air gap due to the mass-inertia of the components. Magnetic forces, which act on the elements of the generator, and vibrations of the generator also influences the width of the air gap during the generator is at operation or at work.
The two bearing arrangement is replaced by the so called “single bearing arrangement”. This technology is disclosed by the document US 2006/0152014 A1 and by the document WO 02/057624 A1 for example. A stationary inner bearing part is attached to a stationary inner shaft and a rotating outer bearing part supports the rotor of the direct drive generator.
FIG. 4 shows the “one-bearing” arrangement. A wind turbine 401 comprises a direct drive generator 402, which is arranged on the upwind side of a tower 403 of the wind turbine 401.
A tower flange 404 is arranged on the top of the tower 403. A bedplate 405 is attached to the tower flange 404. The wind turbine 401 comprises a yaw system—not shown here—which is used to turn the bedplate 405 of the wind turbine 401 around the axis Y.
The wind turbine 401 comprises a stationary shaft 406, while the shaft 406 has a centre axis A. The rear side of the stationary shaft 406 is attached to a retaining arrangement 407. On the front side of the stationary shaft 406 a stator arrangement 408 of the direct drive generator 402 is arranged.
The stator arrangement 408 comprises a stator support structure 409 and a lamination stack 410. The lamination stack 410 supports windings 411.
The stator support structure 409 comprises two support elements 412 for a two side support of the lamination stack 410. The support elements 412 are ring-shaped. They are attached to the outside of the stationary shaft 406.
A hollow cylindrical support element 413 is attached to the outer ends of the ring-shaped support elements 412. The hollow cylindrical support element 413 carries the ring-shaped lamination stack 410 and the windings 411.
A rotor arrangement 414 is arranged around the stator arrangement 408. The rotor arrangement 414 comprises a front endplate 415 and a cylinder element 417. The front endplate 415 is ring-shaped, while the cylinder element 417 is hollow. The cylinder element 417 comprises a plurality of permanent magnets 418, which are mounted on the inside of the hollow cylinder element 417. The permanent magnets 418 are arranged opposite to the lamination stack 410 and the supported windings.
An air gap 419 with a width of approximately 5 mm is located between the permanent magnets 418 and the lamination stack 410. The front endplate 415 is arranged on the stationary shaft 406 via a bearing 420. The bearing 420 is capable to transform axial loads in both directions of the centre axis A. An appropriate bearing is disclosed in DE 201 16 649 U1 for example.
The stationary part 421 of the bearing 420 is attached to the stationary shaft 406. The rotating part 422 of the bearing 420 is connected to a mounting ring 423. The front endplate 415 as well as the hub 424 are attached to the mounting ring 423. The hub 424 comprises mounting devices 425 for wind turbine rotor blades—not shown here. As shown the air gap 419 is uniform, thus a constant distance is established between the elements of the rotor and the elements of the stator.
This design is very attractive as only one bearing is used to support the rotor arrangement. On the other side the single bearing arrangement shows the same drawbacks cited above.